The world wide web, Internet, and many other distributed networks continue to provide opportunities for new and improved digital information dissemination. Digital audio, video, and images may be easily distributed, reproduced, and manipulated. However, these efficiencies also increase the problems associated with copyright enforcement. For this reason, creators and distributors of digital data are hesitant to provide access to their intellectual property. Reliable solutions to the problems associated with copyright protection of multimedia data are actively being pursued.
Digital watermarking has been proposed as a means to identify the owner or distributor of digital data. Watermarking is the process of encoding hidden copyright information in digital data by making small modifications to the data samples. Unlike encryption, watermarking does not restrict access to the data. Once encrypted data is decrypted, the media is no longer protected. A watermark is designed to permanently reside in the host data. When the ownership of a digital work is in question, the information can be extracted to characterize the owner.
A digital watermark is designed to be perceptually and statistically invisible with the host media (e.g., image, audio, or video). This ensures that the watermark does not degrade the host media. It also helps to prevent illegal removal of the copyright protection by a "pirate." The watermark is also designed to be robust to signal distortions, incidental and intentional, applied to the host data. Possible distortions include normal signal processing operations, e.g., coding, filtering, scaling, etc., and deliberate attempts to forge, remove, or invalidate the watermark. Generally, a resourceful pirate may use a variety of signal processing operations to attack a digital watermarking. A pirate may attempt to defeat a watermarking procedure in three ways: (1) damage the host media to make the watermark undetectable, (2) establish that the watermarking scheme is unreliable, i.e., it detects a watermark when none is present.
Finally, (3) the extracted watermark must also correctly identify the owner and solve the deadlock issue when multiple parties claim ownership of a digital work. A deadlock may occur when a second party watermarks another person's data, thereby asserting unlawful ownership of another's intellectual property. That is, the main function of a watermarking algorithm is to unambiguously establish and protect ownership of data. However, many current watermarking schemes are unable to resolve rightful ownership of digital data when multiple ownership claims are made, i.e., when a deadlock problem arises. The inability to deal with deadlock is independent of how the watermark is inserted in the digital data or how robust it is to various types of modifications.
Watermarking techniques which do not require the original (non-watermarked) signal are the most vulnerable to ownership deadlocks. A pirate simply adds his or her watermark to the watermarked data. The data now has two watermarks. Prior art watermarking schemes are typically unable to establish who watermarked the data first.
Watermarking procedures that require the original data set for watermark detection also suffer from deadlocks. In such schemes, a party other than the owner may counterfeit a watermark by "subtracting off" a second watermark from the publicly available data and claim the result to be his or her original. This second watermark allows the pirate to claim copyright ownership since he or she can show that both the publicly available data and the original of the rightful owner contain a copy of their counterfeit watermark.
It would seem that the original (non-watermarked) media should be able to resolve the deadlock issue. Party A should have an original S.sub.orig which does not contain Party B's watermark. On the other hand, Party B's "original" must have Party A's watermark, since it is derived from the data Party A watermarked and distributed. However, current watermarking techniques are susceptible to an intelligent attack by Party B which destroys this logic. In particular, Party B can create a watermark W.sup.B which shows up in Party A's original S.sub.orig. Both originals, one from each party, contains the others watermark. Thus, a deadlock is created.
Party B's watermark is created by "subtracting off" a second watermark from the publicly available data. The difference is declared Party B's "original." For example, suppose Party A watermarks data S.sub.orig.sup.A using their watermark W.sup.A, and allows the watermarked data ##EQU1##
to be accessible to the public. Party B takes the watermarked data S.sub.water and creates their own "original" data S.sub.water.sup.B by subtracting off a second watermark W.sup.B : ##EQU2##
Thus, both watermarks W.sup.A and W.sup.B exist in the publicly available data S.sub.water.sup.A : ##EQU3##
When S.sub.water.sup.A is tested for W.sup.A and W.sup.B, both will be positively identified. The originals from each party may be consulted. Party A can find their watermark W.sup.A in Party B's original as ##EQU4##
However, Party B can find their watermark W.sup.B in Party A's original, as ##EQU5##
As a result, this second watermark allows the pirate to claim copyright ownership since he or she can show that both the publicly available data and the original of the rightful owner contain a copy of their counterfeit watermark. Thus, there is a need for watermarking procedures applicable to digital data that do not suffer from the described shortcomings, disadvantages and problems.